Spasmodic Dysphnia (SD) is a poorly understood, highly controversial voice disorder whose effects are frequently devastating to the lives of its victims. While a number of treatments have been reported to produce some degree of improvements in some patients, for most it remains an intractable, irreversible, profoundly handicapping disability. The question of which patients can be helped by a particular treatment modality remains unresolved. Hypothesized etiologies include (1) psychogenic, (2) neurologic, (3) both psychogenic and neurologic, with no established means of differential diagnosis. Proposed sites of lesion in neurologic theory include peripheral nerve, basal ganglia, brain stem, and cortex. Proposed herein is a systematic investigation of 30 SD patients with respect to (1) objective evaluations of selected neurological functions including visual, auditory, and vagal-visceral, (2) reported and diagnosed non-phonatory symptoms, including course of the disorder and response to treatment(s), and (3) patterns of respiratory, phonatory, and articulatory dysfunction. Inferential and correlational analyses of the results of these studies will yield insight into etiology, probable site(s) of lesion(s), and differential diagnosis.